Head-up displays have long been used in airplanes to aid the pilots. The head-up display reflects information the pilot needs off of a partial mirror or the windshield, referred to as the combiner, and projects that information at a distance in front of the plane so that the pilot does not have to look into the cockpit to see the information.
More recently, head-up displays have been used in automotive applications. In many of these automotive applications, the image is projected up and reflected off of the vehicle front windshield to appear at a distance in front of the driver. With a head-up display in a vehicle, the driver does not have to adjust his or her eyes from the road to read information, such as vehicle speed, which is normally displayed in the vehicle instrument panel.
Additional information which may be included in the projected display is instrumentation telltale information. Telltale information typically indicates to the vehicle operator the state of different vehicle systems, e.g., the headlight high beams being on, or indicates to the vehicle operator the states of various vehicle parameters, e.g., low fuel, low oil pressure, etc..
The optics for many head-up display systems magnify the projected image, which is the projection of an image source. For implementations in which the projected image is magnified, the image source may be miniature in size, while still projecting a large readable image for the vehicle operator.
In some head-up display systems, the brightness of the projected image is enhanced by projecting the image on a treated combiner or windshield. However, treating the windshield enhances the cost of manufacture of the vehicle. To eliminate the need for a treated windshield, the image source must be made sufficiently bright so that the projected image remains visible to the vehicle operator even on bright sunny days.